Raman spectroscopy is an effective tool for identifying and characterizing a vast array of substances.
In Raman spectroscopy, laser light of a known wavelength (typically infrared or near infrared) is directed at a specimen. The laser light (also sometimes referred to as the Raman pump) interacts with the electron clouds in the molecules of the specimen and, as a result of this interaction, experiences selected wavelength shifting. The precise nature of this wavelength shifting depends upon the materials present in the specimen. A unique wavelength signature (typically called the Raman signature) is produced by each specimen. This unique Raman signature permits the specimen to be identified and characterized. More specifically, the spectrum of light returning from the specimen is analyzed with a spectrometer so as to identify the Raman-induced wavelength shifting in the Raman pump light, and then this wavelength signature is compared (e.g., by a computer) with a library of known Raman signatures, whereby to identify the precise nature of the specimen.
Raman spectroscopy is widely used in scientific, commercial and public safety areas.
Recent technological advances have made it possible to significantly reduce the size and cost of Raman spectroscopy systems. This has in turn increased the range of practical applications for Raman spectroscopy. For example, portable units have recently become available for various field uses, such as the on-site identification of potentially hazardous substances.
In some instances it can be desirable for the Raman spectroscopy system to have its optical probe head (i.e., the light launch and collector portions of the system) separated from the main body of the Raman system (e.g., the laser, spectrometer, computer, etc.). For example, this can be useful in situations where the entire Raman spectroscopy system will not conveniently fit adjacent to the specimen. In this case, the optical probe head is typically connected to the main body of the Raman system by a flexible optical connector.
Additionally, in some instances it can be desirable for the optical probe head to be disposable. For example, where specimen purity is of concern, or where the specimen may be highly toxic, it may be desirable for the optical probe head to be replaced after use.
As noted above, where the Raman spectroscopy system has its optical probe head separated from the main body of the Raman system, the optical probe head is typically connected to the main body of the Raman system by a flexible optical connector. This flexible optical connector generally comprises a long, thin light guide, or guides, to deliver the excitation light to the specimen and to collect the Raman signature from the specimen. It is also generally desirable that the light guide, or guides, be flexible, rugged, and compact.
Conventional optical fibers have traditionally been used as the light guide(s) to deliver the excitation light to the specimen and to collect the Raman signature from the specimen. However, the amorphous nature of the glass used in a conventional optical fiber (and, specifically, the non-linear coefficients of the optical fiber's refractive index), together with the high optical power of the pump laser, typically causes a relatively broadband spurious background noise signal, having a significant intensity, to be generated as the pump light travels through the fiber.
As a result, where a single conventional optical fiber is used to both deliver the pump light to the specimen and to collect the Raman signature from the specimen, the relatively broadband spurious background noise signal from the fiber is superimposed on the Raman signature of the specimen. See FIG. 1. Since the relatively broadband spurious background noise signal from the fiber tends to encompass the wavelengths associated with the specimen's Raman signature, the Raman signature is effectively obscured to some extent against the noise of the relatively broadband spurious background noise signal from the fiber. In other words, the signal-to-noise ratio of the Raman signature is effectively reduced, leading to a decrease in the sensitivity of the system and hence a reduction of specimen selectivity.
For this reason, one optical fiber (the excitation fiber) is generally used to deliver the excitation light to the specimen, and another optical fiber (the collection fiber) is generally used to collect the Raman signature from the specimen. See FIG. 2. A filter is generally placed at the end of the excitation fiber to remove the relatively broadband spurious background noise signal induced by the high intensity pump light passing through the excitation fiber. The Raman signature coming off the specimen is picked up by the collection fiber and passed to the optical spectrum analyzer (i.e., spectrometer, computer, etc.). However, the light coming off the specimen and passing through the collection fiber does not induce a significant relatively broadband spurious background noise signal due to the diminished intensity of the light. Thus, by using one conventional optical fiber (and filter) to deliver the pump light to the specimen, and a separate conventional optical fiber to collect the Raman signature from the specimen, the Raman signature remains relatively distinct and readable. The two fibers may be geometrically separated (as shown in FIG. 2) or, alternatively, the geometry can be collinear and an arrangement of dichroic beamsplitters and filters may be used to separate the signal and excitation light.
In any case, the complexity of a two fiber and filter construction (i.e., excitation fiber, filter and collection fiber) increases the cost of the optical probe head and makes a disposable optical probe head significantly less feasible.
Accordingly, a primary object of the present invention is to provide an improved Raman spectroscopy system which overcomes the aforementioned shortcomings of currently available systems.
Another object of the present invention is to provide an improved optical probe assembly which is configured so as to avoid generating a significant relatively broadband spurious background noise signal when delivering the pump light to the specimen.